THE EJECTION-SWEEP CHARACTER OF SCALAR FLUXES IN THE UNSTABLE SURFACE LAYER

Abstract

In the atmospheric surface layer, it is widely accepted that ejection andsweep eddy motions, typically associated with coherent structures, areresponsible for much of the land-surface evaporation, sensible heat, andmomentum fluxes. The present study analyzes the ejection-sweep propertiesusing velocity and scalar fluctuation measurements over tall natural grassand bare soil surfaces. It is shown that momentum ejections and sweeps occurat equal frequencies (D eject ≈ D sweep ≈ 0.29) irrespective of surfaceroughness length or atmospheric stability conditions. Also, their magnitudesare comparable to values reported from open channel velocity measurements (Dsweep ≈ 0.33; D eject ≈: 0.30). The scalar D eject is constant andsimilar in magnitude to the momentum D eject( ≈ 0.29) over both surfacesand for a wide range of atmospheric stability conditions, in contrast to thescalar D sweep. The scalar sweep frequency is shown to depend on the scalarskewness for the dynamic convective and free convective sublayers, but isidentical to D eject for the dynamic sublayer. The threshold scalar skewnessat which the D sweep dependence occurs is 0.25, in agreement with theaccepted temperature skewness value at near-neutral conditions. In contrastto a previous surface-layer experiment, this investigation demonstrates thatthe third-order cumulant expansion method (CEM) reproduces the measuredrelative flux contribution of ejections and sweeps (ΔS0) for momentumand scalars at both sites. Furthermore, a linkage between ΔS0 and thescalar variance budget is derived via the third-order CEM in analogy tomomentum. It is shown that ΔS0 can be related to the flux divergenceterm and that such a relationship can be estimated from surface-layersimilarity theory, and the three sublayer model of Kader and Yaglom andproposed similarity functions.